Parathyroid Hormone: Structure and Immunoheterogeneity

Abstract

The amino acid sequence of the NH₂-terminal 34 residues of human parathyroid hormone (HPTH) has been determined and duplicated synthetically to produce a peptide that is biologically active. In the amino acid sequences of the bovine and porcine hormones, the glutamic acid function at position 22 has been revised to glutamine. Among these initial 34 residues, HPTH differs from bovine PTH by 5 residues and from porcine PTH by 4 residues.

A synthetic preparation of HPTH 1-34 was used to characterize two antisera to PTH. One antiserum (CH 14M) reacted almost equally well with the synthetic peptide (HPTH 1-34) as with HPTH 1-84 obtained from natural sources. The other antiserum (GP 1M) did not react with the synthetic peptide (HPTH 1-34) but reacted with HPTH 1-84 to an identical degree as antiserum CH 14M. On this basis, antiserum CH 14M was designated “anti N” to describe its NH₂-terminal specificity, and GP 1M was designated “anti C” to describe its COOH-terminal specificity.

Radioimmunoassays using these antisera were systematically applied to the measurement of serum immunoreactive PTH (iPTH) in patients with primary or secondary (renal) hyperparathyroidism. The results showed that the “anti C” assay system was superior to the “anti N” assay system in assessing both chronic parathyroid hyperfunction and the biologic effects of excess circulating PTH (osteoclast number in bone biopsies). In contrast, reference is made to the superiority of the “anti N” assay system in assessing acute changes in parathyroid function and in localizing abnormal parathyroid tissue by the method of differential catheterization of the venous drainage of the neck and mediastinum.

An explanation for these important differences in the results obtained with the two assay systems is based on observations that the concentration of COOH-terminal fragments and the serum survival time of these fragments are much greater than those of the native, secreted whole molecule, PTH 1–84.

During the past 5 years there has been a great increase in our understanding of the chemistry, biosynthesis, secretion, and pathophysiology of parathyroid hormone (PTH). PTH has been shown by HAMILTON, COHN, KEMPER, and colleagues (HAMILTON et al., 1971; COHN et al., 1971; COHN et al., 1972; KEMPER et al., 1972) to be synthesized as a precursor with a molecular weight of approximately 12,000. Structural studies of the bovine proparathyroid hormone (pro-PTH) have shown six additional amino acid residues attached to the NH₂-terminal end of the molecule (HAMILTON et al., 1973). Because amino acid compositional data suggest that pro-PTH has amino acid residues not yet accounted for by this hexapeptide sequence, the possibility that there is an additional sequence of amino acids attached to the COOH-terminal region is now being considered (HABENER et al., 1973; COHN et al., 1974). It is likely that the pro-PTH is converted, in the parathyroid gland, to the storage form of the hormone, an 84 amino acid polypeptide with a molecular weight of 9,500. It is not known presently if pro-PTH is released into the circulation.

The 84 amino acid polypeptide is a major form of the hormone secreted by the gland into the circulation after appropriate physiologic stimuli (HABENER et al., 1971). However, recent studies by a number of groups (HABENER et al., 1973; BERSON and YALOW, 1968; ARNAUD et al., 1970; ARNAUD et al., 1971a; CANTERBURY and REISS, 1972; HABENER et al., 1972; SEGRE et al., 1972; ARNAUD et al., 1973; SILVERMAN and YALOW, 1973; ARNAUD,1973a; ARNAUD et al., in press; SEGRE et al., in press) indicate that there are multiple forms of the hormone in the blood of patients with hyperparathyroidism. These multiple forms include the 84 amino acid polypeptide and fragments of it. The major fragment has a molecular weight of 6,000 to 7,000, is COOH-terminal, and has a very long half-life. Canterbury et al. (1973) have demonstrated that a fragment(s) in the plasma of hyperparathyroid patients was biologically active in the renal adenylate cyclase system, which suggests that some of the circulating plasma fragments retain biologic activity. Indisputable immunologic evidence of this latter observation is still lacking, although it is supported by work, presented in the present paper, using an antiserum of great specificity for NH₂-terminal PTH.

The present paper describes recent studies on the chemistry of human, bovine, and porcine PTH as well as some partical consequences of the immunoheterogeneity of PTH in serum when this radioimmunoassay is used in the routine evaluation of patients with hyperparathyroidism.

The work on the amino acid sequence of human PTH could not have been done without the generous donations of human parathyroid tissue to us by more than 150 physicians from 12 different nations. We thank our colleagues Ms. Rosemary Ronan and Drs. Thomas Fairwell, Glen W. Sizemore, and Francis P. Di Bella who actively contributed to the work on the amino acid sequence of human PTH and Dr. Werner Rittel whose group at the Ciba-Geigy Co. synthesized human PTH 1–34. Dr. E. Travis Littledike provided the porcine parathyroid hormone for sequence analysis. Dr. Ralph S. Goldsmith was actively involved in the work on the application of region-specific radioimmunoassays of PTH to the study of parathyroid function in hyperparathyroid patients. We are especially grateful to Dr. Philippe J. Bordier who provided the osteoclast counts and serum for PTH analysis from his patients in Paris. Dr. Thomas Dousa generously carried out the renal cortical membrane adenylate cyclase assays of synthetic bovine and human PTH 1–34. The constantly superb technical assistance of Ms. Julianna Gilkinson, Ms. Judith A. Larsen, Ms. Kathleen A. Safford, and Ms. Judith M. Verheyden is greatly appreciated.